1. Field of the Invention
The present invention relates to injection molding of plastic resins at very high injection pressures and, more particularly, a bimetallic cylinder capable of withstanding injection pressures in excess of 40,000 psi in injection molding processes for producing plastic articles.
2. Description of the Related Art
In the past five years, the plastics industry has seen a marked increase in the use of engineering grade resins that require processing pressures above 40,000 psi. Conventional bimetallic cylinders designed to provide wear resistance have the shortcoming of not being able to withstand injection pressures above 25,000 psi without the addition of a high pressure, heat treated sleeve shrunk fit on the output end of the barrel
While this fix works in most cases up to 30,000-35,000 psi, there is a limit to the pressure these types of barrels can withstand. Conventional barrels will fail at internal injection pressures of around 42,000 psi. In an injection process, the maximum operational pressure is much lower because the process necessitates a safety factor of 1.5. In contrast, the barrel of the present invention did not crack at a pressure of 119,000 psi (limit of testing equipment). Thus, this invention can provide bimetallic cylinders that have the required abrasive wear resistance while providing the capability of sustaining high internal injection pressures. The resultant internal hoop stress levels caused by these high injection pressures can be tolerated with a safety factor of 1.5 minimum and without the need for a high pressure sleeve. It is desirable to eliminate high pressure sleeves not only from a cost point of view, but also due to the fact that plastic can be forced between the high pressure sleeve and the barrel outer diameter which can cause the barrel to crack or to collapse and seize the screw.